This invention relates to a process of forming a metal or metal compound coating on a face of a heated glass substrate during its displacement in a given direction by contacting such face at a coating station through which the substrate passes, with at least one stream of droplets comprising a substance or substances from which said coating metal or metal compound is formed on said face. The invention also relates to apparatus for use in carrying out such a process.
Processes of the above kind are employed for forming coatings which modify the apparent colour of the glass and/or which have some other required properties in respect of incident radiation, e.g. an infra-red-reflecting property.
When forming a coating on a moving glass substrate by spraying, problems are encountered in controlling the thickness of the coating so that it complies with given standards. The thickness of the coating forming on any region within the area of the substrate is susceptible to the influence of various factors. These include the spraying rate, the direction of the spray in relation to the direction of movement of the substrate, and the temperature conditions at that region when it is coated.
It is known to discharge the droplets as a stream which is inclined downwardly towards the substrate and in the general direction in which the substrate is moving. Direction of the spray in that way has been found to be most beneficial for promoting steady conditions at the zone of impingement of the droplets on the substrate. But of course direction of the spray in that manner cannot in itself ensure a deposit of a given thickness at a given region of the substrate, even if the rate of delivery of material through the sprayer is accurately controlled.
The temperature conditions under which any given region of the substrate is coated depend inter alia on the temperature of that region of the substrate when it is sprayed. A relatively high substrate temperature implies a relatively thick coating. Proposals have previously been made to control the temperature of the substrate so that each region will be at an appropriate temperature when it is exposed to the spray, but so far such proposals have not led to satisfactory results. Adjustments of the temperature of the substrate, and in particular its temperature profile transversely of its path of movement through the coating station, could not be made with sufficient speed and accuracy for purposes of industrial production. For similar reasons it has been found to be unsuitable to exercise control of the coating thickness merely by controlling the temperature of the coating precursor material fed to the sprayer.